You wouldn't be the first employee to wonder whether you might find a better job with another firm or company—perhaps the pay will be better, the job duties more in line with your skills, or you want to escape a tyrannical boss. But if you are a foreign employee waiting for a decision to be made on your green card application, now might not be a good time for such a move.
First, let's be clear on what part of the process we're talking about. We're not addressing the initial phases of qualifying for a green card, when an employer has offered you a job and is seeking labor certification on your behalf. That's less likely to create problems (though you'll need to keep a careful eye on the terms and expiration date of whatever visa allows you to stay in the U.S., so that you don't fall out of status).
What this article primarily addresses is the situation of someone legally living and working in the U.S. who has already applied for "adjustment of status" (a green card) with U.S. Citizenship and Immigration Services (USCIS) based on employment. (This is done using USCIS Form I-485.)
Such a person would have either already received USCIS approval of, or have submitted as part of the adjustment application, an I-140 petition (the immigration form employers use to petition someone for an employment-based green card). Such a person would have a work permit and status in the U.S. based on being an "adjustment applicant," and be awaiting a USCIS interview or decision.
Once you have filed for adjustment, and your case is pending, you'd be putting your application at risk if you switched jobs without following applicable rules.
Let's understand the basic concept of employment-based green card sponsorship first. An employment-based green card is based on the idea that a future job has been offered to the foreign worker. That's the legal concept even if that person is already working for the employer and sponsor. The employer offers a "permanent" position, which is set to start upon approval of the green card.
Similarly, the employee promises to accept that position upon approval of the green card. Thus, such employees are expected to work for the employer that sponsored them for a reasonable length of time after receiving green card approval. (There's no set amount of time required.) Failure to do so can create serious problems for these employees later, particularly if they later attempt to naturalize to U.S. citizenship.
So, if failing to work for the sponsoring employer for a reasonable duration after the I-485 is approved could cause serious problems, logic follows that changing employers before the I-485 is approved could cause even more problems. In fact, it could lead to denial of your I-485, unless you fall into one of a few limited exceptions found under the portability rules of the American Competitiveness of the Twenty-First Century Act or AC21, discussed below.
The main issue is that, once USCIS discovers that you are no longer employed by the original employer/sponsor, it could naturally conclude that you no longer intend to work for that employer in the future. That would defeat the whole purpose of the employment-based green card process, which is for you to work for the sponsoring employer. Unless you intend to return to the sponsoring employer upon approval of your green card, or unless covered by AC21 as discussed below, you'd need to proceed with caution and seek immigration counsel before jumping ship from the green card-sponsoring employer.
And you'd of course need to make sure you have authorization, such as H-1B status, to work for the new employer.
If you have applied for employment-based visas in one of these categories: EB-1 Outstanding Researcher, EB-1 Multinational Manager, EB-2, or EB-3 worker, you might actually be able to switch employers while awaiting your adjustment-of-status decision, if you've been waiting for USCIS action for 180 days or more. However, you must carefully follow the "portability" rules when doing so.
In particular, you must find a job that is the "same or similar" as the one you have. If you fail to comply with the rules; for example, you go from a job as a tech analyst to a job as a restaurant manager; your I-485 application can be denied.
For more information on changing jobs while on one of these type of employment-based visas, please see How Soon After I Start Work With My Green Card Sponsor Can I Switch Jobs?
If USCIS denies your I-485 because you switched employers without either filing for proper work authorization or without complying with the above-described portability rules, this could ruin your chances of applying for a green card; or, for that matter, any other immigrant or nonimmigrant visa; in the future.
Here's an example of why this might be:
Let's say you are in the U.S. with H-1B status and, while awaiting a USCIS decision on your I-485, which was pending less than 180 days based on an approved I-140 from Employer A, you start working for Employer B. Your new employer files a new employment-based I-140 petition for you. What's worse, you have reached the end of your H-1B six-year maximum, and no longer qualify to extend it while your new petition is pending. Your green card application will likely be denied. You cannot, after all, adjust status unless you are already in status. If you were to remain in the U.S. after your H-1B expired, you would have "overstayed" your period of authorized stay and be in the U.S. illegally.
To understand the legal effects of overstaying, read Consequences of Unlawful Presence in the U.S.—Three– and Ten-Year Time Bars.
Consult an immigration attorney before changing jobs if your employment-based I-485 is pending; you'll want to obtain a full analysis to determine the best course of action.